The mean angular number densities of X-ray clusters per square degree, within redshift bins of width Δz = 0.1, for the XXL, DES, and SPT surveys. Error bars contain both the shot-noise and sample-variance contributions, from Eqs. (22) and (27). For DES we consider the mass thresholds M > 5 × 10¹³   h^-1   M_⊙ and M > 5 × 10¹⁴   h^-1   M_⊙ (smaller error bars), and for SPT the mass threshold M > 5 × 10¹⁴   h^-1M_⊙ (larger error bars shifted to the right).